1. Field of the Invention
The present invention relates to a diagnostic apparatus and method wherein a density distribution of a specified proton (generally, hydrogen nucleus) in biological tissue is measured externally from the object examined (i.e., a patient) in a non-invasive manner by utilizing a nuclear magnetic resonance phenomenon so as to obtain information for medical diagnosis.
2. Description of the Prior Art
Such a diagnostic apparatus is described in e.g., U.S. Pat. No. 4,254,778.
The known nuclear magnetic resonance techniques (referred to as "NMR" techniques) will now be described with reference to FIGS. 1 through 5.
A steady magnetic field is generated by an air coil C1 shown in FIGS. 1A and 1B, and a magnetic gradient field is generated by gradient field generating coils C2, C3 and C4 (FIGS. 2 and 3) assembled together with the air coil C1. FIG. 4 shows the fields illustrated diagrammatically in the side elevation in relation to a patient P. A steady field H.sub.0 generated by the air coil C1 is superimposed in advance on a gradient field G.sub.z generated by the coils C2. The gradient field G.sub.z can be obtained by flowing reverse currents through a pair of Helmholtz coils C2 shown in FIG. 2. This coil pair is called a "Maxwell pair". The gradient field G.sub.z has the same direction (z-axis) as that of the steady field H.sub.0 and has a zero magnetic intensity on a central plane (perpendicular to the z-axis) between the pair of coils C2 so that the absolute values of the intensities of reverse field components linearly increase in opposite directions from the above-described central plane along the z-axis (FIG. 4). The patient P is then placed in the resultant magnetic field. A selective exciting pulse H.sub.1 having a proper frequency component is applied to the patient P through a pair of saddle-shaped probe head coils C5. The selective exciting pulse H.sub.1 has a center frequency of 4.258 MHz (corresponding to a magnetic field of 1,000 gausses for a hydrogen nucleus) of a carrier wave and is obtained by amplitude-modulating an RF pulse by a SINC function. When the selective exciting pulse H.sub.1 is applied to the patient P, resonance occurs in a plane region (cross-sectional slice region with respect to the Z axis) wherein a frequency corresponding to a vector sum of the steady field H.sub.0 and the gradient field G.sub.z becomes equal to the frequency of the selective exciting pulse H.sub.1. A gradient field G.sub.R obtained by a sum of vector components of gradient fields G.sub.x and G.sub.y (G.sub.x and G.sub.y are perpendicular to each other and to G.sub.z) respectively generated by the gradient field generating coils C3 and C4 is applied to the slice region (i.e., chosen slice region) where resonance occurs. In this condition, when a free induction decay signal FID (referred to as "FID signal") is measured through the probe head coil C5, this signal corresponds to a signal obtained by Fourier-transforming a projection signal indicating a specific nucleus density distribution in the direction of the gradient field G.sub.R within the slice of the patient P. The direction of the gradient field G.sub.R can be varied within the x,y plane by changing the relative ratio of intensity of the field G.sub.x generated by the coils C3 to that of the field G.sub.y generated by the coils C4. A resultant free induction decay signal FID is subjected to inverse Fourier transformation, thereby obtaining projection signals in various directions in the x,y plane. By utilizing these projection signals, an image indicating the density distribution of the specific nucleus within the slice of the patient P is obtained.
As a method which does not perform image reconstruction as described above, a multi-sensitive point method is proposed and disclosed in U.S. Pat. No. 4,184,110 (1980) wherein an AC current flows through a gradient field coil to vibrate the gradient field, to accumulate FID signals and hence to extract only those signal components which are stable on the central line over a period of time.
In these conventional NMR apparatuses, a signal to noise ratio (referred to as an "S/N ratio" hereinafter) of the resultant FID signal is low, and this is the main reason for low spatial resolution of a tomographic image in medical diagnosis. Noise components mixed in the FID signal are summarized as follows:
(a) external noise (e.g., automobile ignition noise, impulse noise mixed in the AC primary power supply (commercial power supply)); PA0 (b) noise from digital equipment (e.g. noise from a digital computer for processing image reconstruction); PA0 (c) noise generated in an oscillator which serves as a component part of the NMR apparatus to generate a selective exciting pulse; and PA0 (d) noise obtained in such a manner that the noise components in items (a) and (b) is mixed in the oscillator described in item (c) and is amplified, and an amplified noise component is mixed in the FID signal.
In particular, noise in item (d) causes great degradation of the S/N ratio since the oscillator has a high amplification, resulting in a great drawback.